Astrophysical discs

Written in 2004

Since the time of Copernicus we have understood that the planets of the solar system orbit the Sun in nearly circular and coplanar paths. Based on this observation, the German philosopher Immanuel Kant proposed in his Allgemeine Naturgeschichte und Theorie des Himmels (1755) that the planets formed out of a thin disc that surrounded the Sun earlier in its history. In Kant's theory, as in the currently favoured paradigm, particles in the disc followed circular Keplerian orbits around the Sun and aggregated into successively larger bodies that eventually became planets.

Less than a decade ago, such protoplanetary discs were directly imaged for the first time by the Hubble Space Telescope, surrounding young stars in the Orion Nebula (McCaughrean and O'Dell 1996). The first discovery of a planet orbiting an extrasolar main-sequence star also dates from this time (Mayor and Queloz 1995), and more than one hundred examples are now known. The formation of a centrifugally supported disc is understood to be an essential part of the process by which a star forms from a slowly rotating cloud of gas that collapses under its own gravitation.

Discs are also found in close binary stars in which one star captures matter lost by its companion through a stellar wind or an overflow of its Roche lobe. If the recipient is sufficiently compact, the captured matter has too much angular momentum to fall directly on to the star and instead forms an accretion disc around it. Matter is accreted if it loses angular momentum through a torque acting on or within the disc. Systems involving accretion on to a white dwarf include the classical novae and dwarf novae, which have been known for more than one hundred years but lacked a physical explanation until more recently. Those involving accretion on to a neutron star or black hole were first revealed by rocket-borne X-ray detectors in the early 1960s.

Accretion of gas through a disc on to a black hole, but having millions or billions of times the mass of the Sun, powers the intense luminosity of active galactic nuclei and quasars (Lynden-Bell 1969). Spiral galaxies themselves differ from gaseous accretion discs in that the principal, stellar component forms an almost collisionless system that cannot be regarded as a fluid. Furthermore, the time-scale associated with accretion processes in the gaseous component generally exceeds the age of the Universe.

There are many other examples of astrophysical discs. Saturn's rings are extremely thin discs composed of icy 'boulders' up to a few metres in size, which undergo very gentle collisions. Rapidly rotating early-type main-sequence stars, known as Be stars, are surrounded by an equatorial decretion disc that is expelled through the action of torque. Discs are also formed when compact binary stars, consisting for example of two white dwarfs, or a neutron star and a black hole, spiral in and merge as a result of gravitational radiation.

A thin disc of gas in Keplerian orbital motion around a central mass is therefore one of the most characteristic fluid flows in astrophysics. Indeed, the physics of discs is sufficiently general that many theoretical aspects can be investigated without detailed reference to the specific environment in which the disc is found. The review articles by Pringle (1981) and Papaloizou and Lin (1995) provide an excellent introduction to the subject.

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